A variety of studies and proposals have been made to find a good compromise between flight distance and "feel" of golf balls. For solid golf balls comprising a solid core and a cover, one common approach is to construct the core and the cover into multilayer structures for adjusting their hardness and dimensions (including diameter and gage).
For example, U.S. Pat No. 5,439,227 discloses a three-piece golf ball comprising a core, a cover inner layer and a cover outer layer, the cover outer layer being harder than the cover inner layer. U.S. Pat No. 5,490,674 discloses a three-piece golf ball comprising a solid core of inner and outer layers and a cover, the core inner layer being harder than the core outer layer.
While the respective layers of most golf balls define smooth spherical surfaces, the golf balls disclosed in U.S. Pat. Nos. 2,376,085 and 5,692,973 have a core which is provided with outwardly extending protrusions for preventing the core from being offset during injection molding of the cover therearound. The protrusions in these golf balls are substitutes for the support pins used during injection molding. These patents do not attempt to positively utilize the shape effect of support pin-substituting protrusions, but rather intend to avoid incorporation of a distinct material in the cover, by forming the protrusions from the same material as the cover.
Recently, JP-A 9-285565 proposes a two-piece solid golf ball in which the solid core and cover, or adjoining layers of a multilayer solid core or adjoining layers of a multilayer cover are provided with irregularities. When hit, the ball gives a different feel to the player, depending on the direction of external force applied to the ball. This golf ball is improved in feel, but insufficient in flight performance and durability. There is left a room for further improvement.
In the prior art, golf balls are often prepared by injection molding, typically by injection molding a cover around a core. One exemplary injection molding method is described with reference to FIG. 7. A mold 21 includes upper and lower sections 21a and 21b which are removably mated along a parting line P to define in the interior a spherical cavity 22 having a negative dimple pattern. A core 23 of a golf ball to be prepared is placed within the cavity 22 as an insert. The core 23 is supported in place by a plurality of support or knockout pins 24 (four pins in each of the upper and lower sections in the illustrated example). A cover stock or molding material 25 is injected into the cavity 22. The support pins 24 are withdrawn from the cover stock 25 immediately before or simultaneously with the completion of injection of the cover stock. After cooling for solidification whereby the core 23 is enclosed within the cover having a multiplicity of dimples, the upper section 21a is opened and the support or knockout pins 24 are moved upward from the lower section 21b for separating the molded golf ball from the lower section 21b. Then, the molded golf ball is taken out of the mold. In FIG. 7, the mold is provided with gas venting holes 26, in which stationary pins 27 are fixedly received to define gaps 28 therebetween. During molding operation, gases, typically air, are discharged from the cavity 22 to the exterior through the gaps 28, and vent holes 26.
Some consideration is needed when such an injection molding method is applied to a spherical core body in the form of a core enclosed within an intermediate layer having a multiplicity of recesses in its outer surface, more particularly when the spherical core body is supported at the center of the mold cavity by support pins and a cover stock is fed into the cavity. Usually, at least the distal ends of the support pins are carefully configured to minimize the formation of pin marks on the cover. The inventor found that it often occurs that some support pins 24 enter recesses 3 in the intermediate layer 2 around the core 1 while the remaining support pins 24 abut the intermediate layer 2 as shown in FIGS. 5 and 6. Then the spherical core body 4 consisting of the core 1 and the intermediate layer 2 is supported off the center of the mold cavity as best shown in FIG. 6. As a result of off-centering of the spherical core body 4, the molded golf ball becomes of poor quality.